1. Field of the Invention
This invention relates to an output circuit placed at the output stage of a semiconductor integrated circuit, and more particularly to an output circuit for use in a semiconductor integrated circuit required to operate at high speeds and to be immune to switching noises.
2. Description of the Related Art
Referring to the accompanying drawings, conventional output circuits will be explained hereinafter.
FIG. 1 is a block diagram of a plurality of output circuits placed in a semiconductor integrated circuit.
FIG. 2 is a diagram showing a construction of the output circuit of FIG. 1, and FIG. 3 is a diagram showing another construction of the FIG. 1 circuit.
As shown in FIG. 1, a semiconductor integrated circuit usually has a plurality of output circuits 8. When a plurality of input signals supplied to the output circuits 8 change in phase with each other, a large current will flow through a power supply line 9 or 6 to drive the loads connected (not shown) to the output circuits 8. Since inductance is parasitic on each of the terminals, bonding wires, frame, and others of the semiconductor integrated circuit, a large current flowing through the power supply line 9 or 6 causes the power supply potential to fluctuate. This results in the problem that noise occurs in the output signal of an output circuit 8 whose input signal remains unchanged or that the semiconductor integrated circuit malfunctions.
This problem will be described in more detail, referring the output circuit 8a of FIG. 2. When the input signal supplied to the input terminal A rises from a low to a high level, the p-channel MOSFET 1 will turn off, and the n-channel MOSFET 2 will turn on. As a result, current flows from the load (not shown) connected to the output terminal B to the ground terminal GND3, causing the output signal from the output terminal B to change from a high to a low level. At this time, the current flowing through the ground terminal GND3 causes the potential at the ground terminal GND3 to fluctuate, creating switching noises
Conventionally, to reduce switching noises in such an output circuit, an adverse effect of the inductance parasitic on the power supply terminals are alleviated by slowing the turn-on speed of the output transistors to suppress changes in the current flowing through the power supply lines.
With the method of slowing the turn-on speed of the output transistors, however, there arises the problem that the semiconductor integrated circuit operates at lower speeds because a change of the output signal potential is slower than that of the input signal potential.